Densitometers are used to control color in a printed product. Color control is desirable so that a color printed with a printing apparatus matches, for example, the same color displayed on a monitor, or generated by a scanner, camera, or other device, or printed by another apparatus. Color consistency is desirable across consecutive pages of a multi-page printed document, job to job, within the same page, and machine to machine.
Densitometers measure optical density, which is generally proportional to ink thickness. Briefly, a densitometer illuminates an area with light from an optical device and detects the light reflected or transmitted from the area. The reflected light is converted to an electrical signal, for example by a photodetector, and the resultant electrical signal is indicative of the ink thickness.
Printing apparatuses typically employ between 4 and 7 inks. Densitometers accordingly may include a light source and a plurality of filters or a plurality of light sources—each to illuminate, and determine the thickness of, a different ink color. For example, in one known system including a densitometer with a plurality of sequentially drivable light-emitting diodes (LEDs), the LEDs sequentially illuminate a measuring point and the reflected light is processed to determine ink density values. That is, light is received from illumination by each LED, one after another. The need to sequentially illuminate each desired area with a plurality of light sources limits the speed with which several areas can be measured.
Standards have been developed for defining the color spectrum—such as the ANSI T standard. Standards are typically based on illuminating an area with a light source having a particular spectrum, so that the reflected light can be universally interpreted. Accordingly, interference filters are used in one system to match the spectral intensities of the illuminating LEDs to spectral ranges provided for obtaining ink density values. The interference filters add complexity, expense to the system and reduce its reliability.
Further, some state of the art densitometers employ one or more incandescent Tungsten lamps. Generally, the Tungsten lamps add additional cost to the densitometer, and produce excess heat. The excess heat effects the densitometer reading and diminishes the accuracy of a densitometer employing a Tungsten lamp. Tungsten lamps take a certain amount of time to stabilize, which increases the time necessary to take a densitometer reading. The performance of a Tungsten lamp also changes over its lifetime, and aging effects can pose problems for the accuracy of the readings.